The subject of the present invention is that of a tail piece for a projectile or sub-projectile, for example a supersonic sub-caliber projectile.
The function of a tail piece is to ensure the aerodynamic stabilization of a projectile in its trajectory.
Tail pieces are commonly used to stabilize projectiles fired by smooth barrelled or rifled guns of large or medium caliber, or by light, recoilless launchers.
This mode of stabilization is the only one that may be used for projectiles fired by a smooth barrelled gun.
Furthermore tail pieces enable projectiles to be stabilized that have a high length to diameter ratio (greater than 7). In fact, however projectiles can not be gyroscopically stabilized.
These projectiles, usually called fin-stabilized projectiles. comprise a sub-projectile or penetrator placed in a sabot (to the calibre of the barrel) which frees the penetrator upon exit from the barrel. See, for example, patent FR2507765, which describes a large caliber (greater than 75 mm) fin-stabilized projectile fitted with a stabilizing tail piece.
The tail piece ensures the stable flight of the penetrator over very great distances (over 2 km for projectiles fired by 120 mm tank cannons).
Such projectiles stabilized by a tail piece nevertheless spin at a moderate speed (around 80 revs/second for a large caliber projectile). a movement that makes up for the geometric dissymmetries of the projectile and that ensures firing accuracy.
This spin is more often than not obtained by deflecting the fins, or by curvature, or else by suitable machining of the leading and trailing edges of the tail piece fins.
In the event that a fin-stabilized projectile must be fired from a rifled barrel, it must be fitted with a sliding drive band that avoids the projectile from being driven at full spin by the rifling of the barrel.
In fact, an excessive spin rate, for a fin-stabilized projectile, causes its immediate destabilization because of the Magnus strain to which it is subjected on its trajectory.
Patent FR2606869 thus describes a sliding drive band that may be used for a large caliber fin-stabilized projectile.
Despite all the studies that have been conducted up to date, the stabilization of fin-stabilized projectiles still causes problems.
It must thus be noted that, during firing, the projectile always has a certain obliquity with respect to the aerodynamic flow. It is not uncommon for an obliquity to be within the range around 2.degree. to 3.degree., which is caused by the disturbances to which the projectile was subjected in the gun barrel and upon exit form the gun barrel.
In the case of sub-caliber projectiles, the separation of the sabot/penetrator also cause a slight obliquity of the penetrator.
The combination of the spin rate and the obliquity generates a Magnus force on the tail piece of the projectile, a force that is exerted in the normal plane to the plane of resistance, i.e., the plane defined by the longitudinal axis of the projectile and its velocity vector.
This strain generates a torque that downgrades performances in external ballistics and may completely destabilize the projectile. The drop in performance is characterised by an increase in loss of velocity, lower accuracy and a reduction in terminal effectiveness, which may be significant.
The harmful effect of the Magnus force is even more significant when the obliquity and spin rate are high.
The influence of the obliquity is reinforced in the case of long projectiles (for example, supersonic sub-caliber projectiles). The interference drag of the wake (or boundary layer) with the tail piece is in fact greater in this case.
Nevertheless, in order to improve piercing performances, large caliber projectiles are now being manufactured that are as long as possible (a length greater than 30 times the diameter of the projectile).
The Magnus phenomenon is also preponderant when the spin rate of the projectile is high, which is the case of medium caliber projectiles (less than 75 mm), even when using a sliding drive band.